TUESDAY, Feb. 26 (HealthDay News) -- The number of younger women who have been diagnosed with advanced breast cancer has increased slowly, but steadily, since the 1970s, a new study indicates.

Over the past 30 years, the number of cases of metastatic breast cancer in women under the age of 40 has tripled, said Dr. Rebecca Johnson, medical director of the adolescent and young adult oncology program at Seattle Children's Hospital.

"The increase translates to about 250 cases per year in 1976, and 850 in 2009," said Johnson, who is also an assistant professor of pediatrics at the University of Washington, in Seattle.

However, she stressed, the absolute increase was smaller. In 1976, the rate of advanced breast cancer in this age group (25 to 39 years) was 1.5 for every 100,000 women; in 2009, it was just under 3 per 100,000 women.

While the number of cases tripled, the rate only doubled because the base population of women grew over the 30-year period studied, Johnson explained.

The findings are published in the Feb. 27 issue of the Journal of the American Medical Association.

In contrast, Johnson said, no such trend has been found for diagnoses of earlier breast cancer in this younger age group or for diagnoses of all stages of breast cancer in older women.

Johnson couldn't say for sure what is driving the increase, as the study only looked at the number of women diagnosed with advanced disease over time. What is needed next, she said, is research to figure out what is fueling the trend.

"Young adults are the least likely to have medical insurance of all age groups," she noted, so that complicates the picture. Breast cancers in younger women also tend to be more aggressive than in older women.

Younger women often believe breast cancer can't happen to them, said Johnson, who had breast cancer in her 20s. When a younger woman does seek medical help for worrisome breast symptoms, she should expect the doctor to take the symptoms seriously and not suggest a prolonged period of watching and waiting. Typically, a doctor should schedule an ultrasound or mammogram screening, Johnson said.

The findings are not a reason to change current mammography screening guidelines, Johnson added. Many organizations recommend routine screenings beginning at age 40, although the U.S. Preventive Services Task Force now recommends routine mammogram screenings need not begin until age 50.

The finding does stress the importance of younger women being aware of breast changes and the importance of seeking medical attention when they find such changes, she said.

For the study, Johnson and her team looked at data from the U.S. National Cancer Institute Registry.

The researchers looked at information about whether the cancer was localized or had spread to organs, bones, brains or the lungs. The increase was found among all races and ethnicities from 1992 (when that data was first available) onward.

Johnson began the research when she noticed she was hearing from friends and other sources about how many more younger women were being diagnosed with late-stage breast cancer.

Other studies should be done, she said, to verify what she has found.

Other cancer experts urged caution when interpreting the study results.

"It's an interesting finding and an important finding, but it has to be put into perspective," said Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society. The increasing trend of advanced breast cancer in younger women has been consistent over time, he noted.

He acknowledged that a diagnosis of advanced breast cancer at any age can be devastating. But experts need to continue to monitor the trend of advanced breast cancers in younger women, he added.

Dr. Courtney Vito, a staff surgeon of general oncologic surgery at the City of Hope Comprehensive Cancer Center in Duarte, Calif., agreed. "The number of women [aged] 25 to 39 presenting with advanced breast cancer or any kind [of breast cancer] still remains relatively low, especially compared to women 40 and older," she said.

Lichtenfeld said the finding "emphasizes the importance of breast self-awareness, that no one knows your body better than you do."

Some younger women with symptoms may be dismissed by the doctor who sees them, Vito said. "It is not unusual for a woman who is 25 to 39 with a breast lump to have sought consultation with multiple doctors before getting an appropriate work-up," she said.